Rapid yield degradation detection in modern fabrication facilities is important. Identifying the cause cuts the losses suffered from process and equipment failure and helps improve profitability. Yield losses are usually classified as either random yield losses or systematic yield losses. Systematic yield losses usually result in a spatial signature that can be observed on an escort wafermap. Early detection of the spatial signature is important as the derogation can quickly spread from a small area to a large area. One method of detection is to manually review the spatial signature. The difficulty with this method is that it is time consuming and is based on the subjective judgement of an engineer. In particular the differing subjective judgement can result in early triggering, non-triggering, late triggering and in particular inconsistent triggering.
U.S. Pat. No. 5,982,920 to Tobin et al. describes a method to automatically detect defects in spatial signatures the system described relies on visual grouping and shape analysis and therefore is complicated to implement.
Other problems in detecting degradation include the non-linear process manufacturing nature of wafer fabrication adding to the complexity of identifying degradation and the different volumes produced of different products.